Comparison of methods for fluoride extraction from forest and cropped soils in vicinity of an aluminum smelter in galicia (NW Spain)

Abstract

Four extracting solutions (1 M NH₄Cl, 0.02 M NH₄Cl, 0.01 M CaCl₂, and water) have been tested to determine mobile fluoride in forest and cropped soils having high fluoride contents resulting from emissions by an aluminum smelter in Galicia, NW Spain. Seventy-two acid forest soil samples and 47 cropped soil samples were analyzed. Linear regression analysis was performed by SPSS 9.0 statistical package. Significant correlations (P < 0.01) were found between fluoride concentrations in different extracts provided that soils had similar pH and exchange aluminum (Al) and calcium (Ca) values. The 1 M NH₄Cl, 0.02 M NH₄Cl, and 0.01 M CaCl₂ extractants were more efficient than water in extracting fluoride from acid forest soils, due to displacement of exchange aluminum and, therefore, Al-bound fluoride. Highest extractions were achieved by 1 M NH₄Cl, due to its higher ionic strength. Water extracted more fluoride than salt solutions from cropped soils, usually fertilized and limed, and consequently having high (>10 cmole_c kg⁻¹) exchange Ca values. When extracting cropped soils by NH₄Cl and CaCl₂, the CaF₂ solubility product is presumably reached due to displacement of exchangeable calcium, and fluoride is removed from solution. Water extractions seem adequate to estimate mobile F from soils high in exchange Ca (>10 cmole_c kg⁻¹), while salt solutions seem suitable for soils high in exchange Al (>1 cmole_c kg⁻¹).

Acknowledgments